Are you referring to the EXS24 piano/keyboard sounds or the ES2 piano keys/sounds? My guess is probably both
I know what you mean about being a bit underwhelmed with the keys after awhile – I started having a similar issue. However, I began to think of those default piano sounds as just the "draft" of the overall sound I'm trying to achieve. I find that adding effects (such as a properly mixed bitcrusher, izotope's FREE vinyl plugin, ring modulator, speaker rotary, delay) add the right amount of warmth and/or grime that I'm looking for to change up the sound's color a bit. Also, reverb experimentation (even the EnVerb) can be quite effective, though I'd recommend pluggin the reverb in a bus and then sending the piano to that bus for maximum control.
If you haven't already, try this in the ES2: loading up a preset piano/keyboard and mess around a bit with the macro controls at the bottom of the interface - it's a quick and effective way to dramatically change up the timbre.
As far as optimum frequency range, it's really up to you. If its an intro piano or if you're looking for your keys to sound like a warm pad, you may want to keep the low end warmth by rolling off (or high passing) frequencies below 200-250 to keep them from interfering with the low end while keeping a bit of depth. If you want the piano to act more as a lead sound, I'd probably use the higher octave notes and then EQ to taste. Some people roll of the upper frequcies to get a less sharp sounding keyboard, while some increase the higher frequencies for a bit more sparkle and edge. Also, the EXS24 and ES2 piano sounds seem to be very keen to velocity, so you can always use the transform>random velocity MIDI function to add a varying pressure thoughout the notes for a more "humanized" feel.
Hope this helps.
Cheers.
